1. Field of the Invention
This invention relates generally to crosstalk interferences and transmission quality affected thereby. More particularly, it relates to a system, method, and apparatus that achieves crosstalk cancellation at the transmitter end, at the receiver end, or at both ends with information obtained at the receiver end.
2. Description of the Related Art
Crosstalk generally refers to unwanted noises, sounds or other signals picked up by one channel of an electronic communication system from another channel, for example, between telephone lines in the same or neighboring bundles. Unlike Gaussian and power supply noises, crosstalk cannot be overpowered with large signal swings. On the contrary, crosstalk scales with signal swing, i.e., a larger bandwidth is usually accompanied with more severe crosstalk interferences. Today, degradation of transmission quality by the crosstalk noises remains a significant problem at the network level, for example, Ethernet, DSL, etc., as well as at the device level such as disk drive read-write devices, etc.
Crosstalk is commonly classified into near-end crosstalk (NEXT) and far-end crosstalk (FEXT). FEXT is similar in nature to NEXT. However, FEXT and NEXT affect the transmission quality differently. NEXT affects any systems which transmit in both directions at the same time, for instance, echo-canceling systems. FEXT appears at the far-end, or opposite end, of its source via a communication link such as a cable. NEXT is larger (i.e., dominant) than FEXT because the interference source is closer to the receiver. This is true when the channel of signal path has great attenuation. When the channel is short, however, both NEXT and FEXT have the same degree of effect on the transmission quality.
Many existing systems focus on reducing or mitigating NEXT. For example, some DSL systems use frequency- or time-division duplexing to avoid transmitting in both directions in the same band at the same time. Moreover, because the interference source is closer to the receiver, most of the known techniques perform crosstalk cancellation at the receiver end.
On the other hand, very few viable solutions available today that can effectively cancel FEXT. One of the obstacles is that FEXT tests are affected by signal attenuation to a much greater degree than NEXT, since FEXT is measured at the far end of the communication link where signal attenuation is the greatest. Moreover, measuring FEXT itself is a daunting task. As discussed by J. W. Cook et al. in “The Noise and Crosstalk Environment for ADSL and VDSL Systems,” IEEE Communications, May 1999, pp. 73-78, measuring FEXT is very time-consuming and involved, even in a laboratory environment.
Clearly, there is a need in the art for a viable crosstalk cancellation system that is capable of overcoming crosstalk interferences not only at the near-end but also at the far-end. The present invention addresses this need.